Method for providing environmentally stable aluminum surfaces for adhesive bonding and product produced

ABSTRACT

Environmentally stable bond joints of aluminum metal and aluminum alloys in adhesively joined structures are formed by utilizing a prebonding anodization of the aluminum surfaces in a phosphoric acid electrolyte containing from about 1.5 to about 50% H 3  PO 4  at 1 to 50 volts for a period of 5 to 60 minutes using a bath temperature of 50° to 100° F. The anodized surface is then washed free of electrolyte, dried and coated with an adhesive resin. The aluminum metal segments are then arranged in a composite arrangement and bonded together under pressure and heat to cure the adhesive resin. The resulting structure is resistant to failure of the bond joints on exposure to moist atmospheric conditions. The surface preparation provides a hydration resistant aluminum oxide surface which minimizes adhesive-aluminum separation under aqueous exposure. Alloys containing copper and other alloying constituents may be successfully anodized and bonded by this process.

This is a continuation, divisional of application Ser. No. 440,387,filed Feb. 7, 1974, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to methods of preparing environmentally stablebonded aluminum structure and more particularly relates to methods ofpreparing bonded aluminum structures in which the aluminum surface isrendered especially well adapted to receive the adhesive resin and isresistant to subsequent delamination and failure of the adhesive bond atthe adhesive resinaluminum interface.

It is well known that aluminum or aluminum alloy surfaces exhibitunpredictable and unreliable adherence to bonding media particularly inmoist and salt laden atmospheres. It has been proposed to increaseadherence of surface coating such as electroplated metal on aluminumbase by means of an anodic treatment in an acid bath and then dissolvinga portion of the oxide film in an acid or alkaline bath prior toelectroplating. See U.S. Pat. No. 1,971,761. It has also been proposedto electroplate directly over an oxide film produced by anodizingaluminum or aluminum alloys in chromic acid of phosphoric acid solutionwithout intermediate treatment of the oxide film such as is taught inU.S. Pat. Nos. 1,947,981 2,036,962 and 2,095,519. In each of theabove-noted patents the aluminum surface is being prepared forelectroplating.

Similarly, it has been proposed to form anodic coatings having improvedadhesive properties on aluminum surfaces by depositing coatings on thealuminum substrate by subjecting the aluminum article to electrolytictreatment in an aqueous solution of various acids such as phosphoricacid, oxalic acid, sulphuric acid, malonic acid and the like at elevatedtemperatures for a very short treatment period. Similarly, it is knownto treat oxides already formed on an aluminum surface by other meanswith a phosphate bath electrolysis to render the oxide surface hydrationresistant. The elevated temperature phosphoric acid anodization processresults in the deposition of an oxide surface characterized as"pseudoboehmite", a highly active form of aluminum oxide. Thecharacteristics of this form of aluminum oxide apparently permit failurewithin the oxide structure when high stressed under humid conditions.The treatment of existing oxide films with phosphoric acid anodizationresults in a film which apparently lacks coherency and stability at theoxide-metal interface and as a result is inadequate for the formation ofan environmentally stable adhesive bond.

OBJECTS OF THE INVENTION

Accordingly, it is one object of this invention to prepare adhesivelybonded aluminum or aluminum alloy structures wherein theadhesive-aluminum interface exhibits environmental stability in anaqueous environment.

It is a further object of this invention to provide a method of formingadhesively bonded aluminum composite type structures in which adhesivefailures at the aluminum-adhesive interface are minimized.

It is a still further object of this invention to provide an adhesivelybonded aluminum structure wherein the aluminum surface is subjected to alow temperature anodic electrolysis in a dilute phosphoric acid bathunder conditions which enhance the formation of an anodic porouscoherent oxide while minimizing or eliminating the deposition ofaluminum oxide from the electrolyte.

One specific object of this invention is to provide an anodizationprocess which may be used to prepare aluminum alloys containing copperfor bonding into a structure which is environmentally stable.

SUMMARY OF THE INVENTION

The structural bonding of metal to metal and composite type assemblywidely used in the aircraft industry and elsewhere frequently require aresultant structure which is reasonably resistant to the extremes ofatmospheric conditions found in use. For example, in aircraftconstruction the wing structure utilized in manufacture of largepassenger, cargo, and military aircraft, utilize adhesively bondedstructures which are subjected to extremes of temperature varying fromsubstantially below zero Farenheit in Arctic areas to temperatures inexcess of 150° F. in tropical areas when the aircraft must be exposed tothe tropical sun. Aircraft are also exposed to marine environments andother highly corrosive atmospheres. To avoid failures of th aircraftstructures as well as to meet the stringent requirements of the militaryaircraft standards and the standards established by the aircraftindustry for commercial passenger and cargo aircraft, bonded metal tometal and composite type assemblies must be able to withstand theenvironmental conditions to be encountered. Of particular importance isresistance to corrosion and delamination of composite structuresoccasioned by humid warm environments which attack prior art materials.Heretofore, the adhesively bonded metal-to-metal and composite typeassemblies have performed less than satisfactorily due to adhesivefailure at the interface between the polymeric adhesive and the aluminumsurface, frequently necessitating field repairs and occasionally removalof the aircraft from service so that extensive repairs may beundertaken.

The present invention contemplates the formation of an environmentallystable, oxide coating on the surface of an aluminum object which iswell-suited to adhesion by known polymeric adhesives and resulting in anadhesively bonded structure which upon exposure to severe environmentsmaintains its structural integrity. When highly stressed under severetest conditions, the resultant structure predominantly exhibits cohesivefailure within the adhesive layer rather than adhesive failure at theadhesive-metal interface. The aluminum is prepared by a surfacetreatment to form a porous anodic oxide coating using a phosphoric acidelectrolyte maintained at a temperature in the range of 50° to 100° F.while imposing a potential of from about 3 to about 25 volts for aperiod of about 10 to 30 minutes.

The above-noted processing parameters for the anodizing step aresuitable for anodization of both aluminum alloys and relatively purealuminum metal commonly employed in adhesive bonded structures. Thehigher range of temperatures, i.e., in the range of about 65° F to 95° Fare best employed with aluminum alloys of high aluminum content and forpure or nearly pure aluminum metal. The higher temperature range notedis less satisfactory for aluminum alloys having substantial amounts ofother metals therein such as 7075-T6, 2024-T3 and other aluminum alloymaterals widely used in industry. For these materials the preferredtemperature range is from about 50° F to about 85° F. Similarly forplated and clad alloy aluminum sheet the higher temperature ranges arepreferred.

Anodizing under the conditions disclosed herein consistently produces asurface superior in performance to that produced by conventionalindustry standard methods, such as chromic acid anodizing or sulphuricacid-sodium dichromate etch. This superior performance is clearlydemonstrated by the bond stability test shown in FIG. 8 while exposingthe specimen to different water and salt environments. Conventionallyprocessed 7075-T6 aluminum clad bondements prepared by chromic acidanodization fail at the oxide-primer interface within two to three dayswhen exposed under stress to hot humid conditions. The same alloy,phosphoric acid anodized prior to bonding under the preferredanodization parameters set forth below does not show any evidence ofinterfacial failure after exposure to the same environmental conditionsfor more than 7 months. Typical failure modes of specimens prepared withthe production parameters noted above are cohesive, i.e., the specimensfail in the resin adhesive zone rather than at the adhesive-metalinterface. Thus, interfacial failure modes which typify service failuresare eliminated or at least minimized with this method of aluminumprebond phosphoric acid anodization surface preparation.

Hydration resistance of oxides formed by anodization in phosphoric acidappear to be a significant factor associated with the improvement inbond stability and their low reactivity to water. The applicantspostulate that bonds of aluminum to adhesives which are exposed to waterand then torn apart at the adhesive-metal interface are in realitycohesive failures within the oxide suggesting that most bond failuresexhibiting adhesive failure after exposure to water are due to weakeningin the oxide by hydration. The applicants further postulate that thefailure mechanism associated with adhesive appearing failures of bondedstructure are due to weakening of the oxide by hydration resulting indelamination when the bond is stressed. Once delamination occurs,corrosion can then take place in the delaminated area causing additionaldamage to the bonded structure. The applicants have found thatphosphoric acid anodization of the surface of aluminum metal and alloysusing relatively low temperatures and dilute phosphoric acidelectrolytes eliminates the delaminating mechanism and subsequentcorrosion.

The most significant aspects of low voltage anodization in phosphoricacid of aluminum surfaces prior to adhesive bonding are that the processprovides positive control of the oxide formation and therefore highreliability, thus producing an oxide with desirable physicalcharacteristics which are more stable in the presence of water than areother anodically formed or deposited oxides. The process provides arange of anodizing conditions in which both relatively pure aluminummetal and aluminum alloys commonly used for bonding can be anodized,i.e., 2024-T3 aluminum alloy and 7075-T6 aluminum alloy as well as thosealloys of a higher aluminum content. The process is also well suited totreatment of clad aluminum material.

Temperatures in excess of about 100° F in solutions of phosphoric acidcause the dissolution rate of the oxide layer to exceed the rate atwhich it is formed so that the oxide surface is removed. Substantialamounts of aluminum present in the phosphoric acid electrolyte solution,may cause a deposition of aluminum oxide in another form such as thatdesignated "pseudoboehmite" in U.S. Pat. No. 3,672,972 and U.S. Pat. No.3,714,001. The conditions under which this "pseudoboehmite" depositionoccurs and under which the applicants discovery of the environmentallystable oxide film formed under the conditions taught herein varies withthe temperature, acid concentration and aluminum concentration.Generally, phosphoric acid anodization at temperatures above about 100°F according to the teaching of U.S. Pat. 3,672,972 and 3,714,001, resultin the deposition of "pseudoboehmite" while the phosphoric acidanodization at temperatures below about 100° F result in the applicants'aluminum oxide described herein. At lower temperatures substantialquantities of aluminum may be present in the phosphoric acid electrolytewithout causing deposition of "pseudoboehmite". Under the processingconditions set forth below a columnar-type closely adherent aluminumoxide film is formed by oxidation of the surface of the aluminum oraluminum alloy. This film has a thickness varying from 500 to 6,000Angstroms with pores having a diameter in the range of 300 to 600Angstroms and a depth of about 400 to 5,500 Angstroms extending into thefilm. These pores provide many additional locations for bonding byproviding more surface area and a mechanical interlock between theadhesive and the aluminum oxide.

The above-noted objectives of this invention and the features discussedbriefly in the summary of this invention will become more readilyapparent from a detailed examination of the following discussion of thepreferred embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of two widely used prior artprocesses for preparing aluminum surfaces for adhesive bonding;

FIG. 2 is a schematic flow diagram of the process of this invention;

FIG. 3 is a graph showing sustained stress lap shear test data forbonded structures prepared by one process of FIG. 1 as compared tobonded structures prepared by the process of FIG. 2;

FIG. 4 is a graph similar to FIG. 3 for tests conducted at a lowertemperature;

FIG. 5 is a graph showing crack propagation data for various bondedstructures treated by prior art processes and by the process of thisinvention;

FIG. 6 is a graphical representation of test results of various bondedstructures using prior art surface treatments and variations in theprocess taught herein;

FIG. 7 is a schematic representation of the crack propagation testutilized in evaluating the laminates formed using the process of thisinvention;

FIG. 8 is a graphical representation of the sustained stress lap sheartest used in evaluating the bonded structures formed according to thisinvention;

FIG. 9 is a graphical representation of test results comparing sustainedstress lap shear test data for various methods of pretreatment of thealuminum surface and the resultant effect on adhesive versus cohesivefailure;

DETAILED DESCRIPTION OF THE DRAWINGS

Referring specifically to FIG. 1, two of the well known prior artprocesses are set forth in a step-by-step fashion in which aluminummaterial as received is first subjected to a degreasing and cleaningprocess in preparation for the surface treatment. The alkaline cleanerutilized is removed in a hot water rinse and the surface is thendeoxidized by exposure to a suitable etchant such as sodiumdichromate-sulfuric acid deoxidizer. One widely used deoxidizer oretchant for aluminum is sold by Amchem Products, Inc., Ambler,Pennsylvania, under the trade name "Amchem No. 6-16" to which nitricacid is added. A suitable etchant for aluminum at room temperature hasthe following composition: 4 to 9 percent by volume Amchem No. 6, 10 to20 oz/gal nitric acid in an aqueous solution.

The aluminum is subjected to the above-noted solution at 65° to 90° F.for a period of time sufficient to deoxidize the surface of thealuminum.

In the event that the aluminum as received is reasonably clean and has athin adherent oxide coating, the above-noted steps may be unnecessaryprior to the anodization step.

After the surface has been deoxidized, if necessary, the surface isrinsed with cold water and then subjected to an acid anodization steputilizing chromic acid as the electrolyte. The chromic acid is suitablyof a concentration of about 5% by weight chromic acid in water.

The aluminum surface is subjected to the anodization at 95° F. with anapplied voltage in the range of 40 volts for a period of time sufficientto form an oxide coating of about 20,000 to about 30,000 Angstromthickness. The chromic acid is rinsed from the surface of the aluminumand the aluminum surface is dried in preparation for the application ofthe adhesive materials.

Similarly when the sulfuric acid-sodium dichromate etch process iselected an aqueous solution containing about 4.1 to about 4.9 ounces ofsodium dichromate dihydrate per gallon of solution and about 38.5 to41.5 ounces H₂ SO₄ per gallon is used. The etching process takes placeat a temperature of about 140° F to 160° F.

A suitable epoxy or other primer is used such as a corrosion inhibitingepoxy primer designated as BR127 manufactured and sold by AmericanCyanamide Corporation. This epoxy primer is a 250° F. cure epoxy resinsuitable as a corrosion inhibiting primer for the bare metal surfaces.

An adhesive material such as a modified epoxy resin having suitablecuring characteristics is then applied to the primed aluminum surface.Several modified epoxy resins are readily available and are suitable foruse in this invention including a product designated as FM123-2manufactured and sold by the Bloomingdale Division of AmericanCyanamide; a product designated as AF126 modified epoxy resin having a250° F. cure manufactured and sold by Minnesota Mining and ManufacturingCorporation and the modified epoxy adhesive designated as Hysol 9628manufactured and sold by Hysol Division of Dexter Corporation. Manyother resins are workable as adhesives for this invention. The primedand taped aluminum surfaces are then placed into engagement underpressure and cured at an elevated temperature to effect the joint orbond between the surfaces.

FIG. 2 shows a flow diagram of the process of this invention in whichaluminum as received is subjected to similar cleaning and deoxidinzingsteps as those outlined above for FIG. 1 if they are found to benecessary due to the condition of the aluminum surface. When thepreliminary cleaning steps are completed, the aluminum surface issubjected to a low temperature anodization process in a solution ofphosphoric acid. The following process parameters have been found togive exemplary results in the performance of the resulting bondedlaminate in use:

                  TABLE I                                                         ______________________________________                                        Temperature    Potential Time    H.sub.3 PO.sub.4                             ° F     (Volts)   (Min.)  Concentration                                ______________________________________                                        Usable                                                                        range    50-100    1-50       5-60 1.5-50%                                    Preferred                                                                     range   60-95      3-25      10-30 3-20%                                      Aluminum                                                                      Alloys  50-80      3-25      10-30 3-20%                                      Pure                                                                          Aluminum                                                                              65-95      3-25      10-30 3-20%                                      Most                                                                          preferred                                                                     range   75±10   10±1   20±5                                                                             10±2%                                   ______________________________________                                    

Anodizing under the conditions set forth above consistently produces asurface superior in performance to that produced by conventionalindustry standard methods, i.e., chromic acid anodizing or sulfuricacid-sodium dichromate etch. This superior performance is clearlydemonstrated by the bond stability test technique shown in FIGS. 7 and8, and the resulting test data presented in FIGS. 3, 4, 5, 6 and 9.

EXAMPLE I

Comparative data for the aluminum surface preparation techniques shownin FIGS. 1 and 2 are presented in FIG. 3 for various epoxy resinadhesives used in preparation of a composite structure. All samples wereprepared by cleaning as follows prior to anodizing:

(1) The surface was vapor degreased by exposure to trichloroethylene for3 minutes at 190° F.

(2) the surfaces were then subjected to an alkaline cleaning agent suchas Wyandotte Altrex, manufactured by Wyandotte Chemicals Corporation,Wyandotte, Michigan; Pennsalt A31, manufactured by Pennsalt ChemicalCorporation of Philadelphia, Pennsylvania; or any of the otherwell-known equivalent aluminum cleaners available and known to theindustry. The aluminum surface is exposed to the alkaline cleaner for aperiod of about 10 minutes.

(3) The aluminum surface is then rinsed with hot water for 5 minutes toremove the alkaline cleaning agent.

(4) A prebond etch in the sodium dichromate-sulfuric acid deoxidizernoted above for 10 minutes at 150° F.

(5) the surface is then immersed in cold tap water rinse for 5 minutesto remove the prebond etchant material.

One-half of the samples were then dried and primed with BR127, an epoxycorrosion resistant primer, 250° F. cure, manufactured by AmericanCyanamide. The remaining samples were subjected to an anodization in 3percent phosphoric acid at 75° F. for 10 minutes with an imposed voltageof 5 volts. The surfaces were then washed with a water rinse, dried andprimed with BR127 as noted above.

The 2 groups of samples were then divided into 3 subgroups each andcoated with the following adhesive materials:

                  TABLE II                                                        ______________________________________                                        Designation    Material                                                       ______________________________________                                        FM123-2      Modified epoxy resin adhesive,                                                250° F. cure, manufactured by                                          American Cyanamide,                                                           Bloomingdale Division.                                           AF 126       Modified epoxy resin adhesive,                                                250° F. cure, manufactured by                                          Minnesota Mining and                                                          Manufacturing.                                                   Hysol 9628   Modified epoxy resin adhesive,                                                250° F. cure, manufactured by                                          Hysol Division, Dexter                                                        Corporation.                                                     ______________________________________                                    

The samples were then assembled in a form suitable for use in the testschematically shown in FIG. 8 and subjected to endwise stress of 1,750psi while immersed in 3.5 percent sodium chloride solution at 140° F. Inall cases the samples anodized in phosphoric acid presentedsubstantially superior results to those prepared in the prior artprocess. Of particular interest is the nature of the failure, thosesamples prepared with the prior art process having predominantlyadhesive failure at the interface between the adhesive and the metal,while those manufactured utilizing the process of this invention hadsubstantially less adhesive failure, with the failure beingpredominantly cohesive in the resin itself.

EXAMPLE II

Test results for sustained stress lap shear tests at 2,750 psi while thesample was immersed in 3.5 percent sodium chloride at 75° F. arepresented in FIG. 4 for samples prepared in a manner corresponding tothose described above for FIG. 3. Specimens prepared by prior artetching process failed within one day of the start of the tests. Thosesamples prepared using a phosphoric acid anodization in 3% H₃ PO₄ at 70°F for 10 minutes at 5 volts demonstrated superior resistance to failure.Four out of five specimens bonded with AF126 and all specimens bondedwith Hysol 9628 survived 30 days test without failure.

EXAMPLE III

Table III shows the results of 120° F., 100 percent relative humiditytest for a test specimen prepared as shown in FIG. 7 and indicate theeffect of solution temperature on bond stability for 8% and 12%phosphoric acid solutions. Excellent results were obtained indicatingthat less than three-tenths of an inch of crack growth was encounteredfor both 8 and 12 percent solutions after 60 days of exposure. SpecimenF-1 and F-5 showed a higher degree of adhesive failure for anodizationat 60° F. suggesting that 60° F. is a marginal temperature for theanodization process when the substrate is pure or nearly pure aluminumor clad aluminum.

EXAMPLE IV

In order to determine the optimum process condition, numerous samples of7075-T6 aluminum clad panels, 6 inches square, having a thickness of0.063 inches were prepared using a preanodization process in which thesurfaces of the aluminum panels were exposed to a solution of Amchem7-17 (a proprietary solution containing nitric acid sold by AmchemProducts, Inc., Ambler, Pennsylvania). This solution is a roomtemperature etchant for aluminum. After surface etching with the Amchem7-17 solution, 4 panels per condition noted in Table IV were anodizedand prepared for bonding by spray rinsing the anodization solution fromthe surface and drying the surface at 140° F. for 10 minutes. BR127primer was applied to the prepared surface and the panels were bondedwith Hysol 9628. The epoxy was applied in a 10 mil thick layer. Ten1-inch wide fracture specimens were saw cut from each pair ofassemblies. Six specimens from each assembly were exposed to boilingwater and the amount of crack growth was measured after 1, 4 and 24hours for specimens prepared as shown in FIG. 7. The remaining 4specimens were exposed to 5 percent salt spray at 90° F. and the amountof crack growth was measured. The test results are presented in Table Vfor the water boil test and Table VI for the 5 percent salt spray at 90°F. test.

The average crack propogation rate and failure mode of the specimenssubjected to boiling water indicated less than eight-tenths inch ofcrack growth after 24 hours exposure. Some specimens that were anodizedin 3 percent phosphoric acid at 65° F. for 10 minutes (see specimen A1and A2) showed adhesive failures. All other failures werecenter-of-the-bond or cohesive.

The oxide coating weight varied from 15 mg/ft² to 47 mg/ft². Nocorrelation between the coating weight and the bond stability was found.

The test results for the 5 percent salt spray at 95° F. crack growthdata is shown in Table VI. Extended exposure to the salt environmentinduced failures of more anodized conditions than did the water boiltest. However, since the adherent was clad aluminum alloy the claddingis sacrificial in a corrosive environment and is uncertain if theseadhesive failure modes were the result of galvanic corrosion, less thanoptimum surface preparation or a combination of both.

EXAMPLE V

Variations of the anodization process parameters were explored and theresults shown in Table VII. The initial room temperature lap shearstrength was 5200 ±200 psi and the mode of failure 100 percent cohesivefor all specimens. Under sustained stress of 1750 psi, most of thespecimens failed in 20 t 200 hours. The specimens prepared in 17 percentH₃ PO₄ at 100° F. and 3 volts anodizing potential (Test A6) showed poorbond stability and failed in less than 23 hours with 40 to 50 percentcohesive failure. The processing conditions of this test would appear tocause excessive oxide dissolution during anodization and not permit thebuild-up of the desired type of oxide coating. The high temperature thusresults in a poor oxide film formation and resultant poor bondperformance.

Corresponding specimens to Test A6 when tested in a sustainedstress/fracture test had complete separation of adhesive from thealuminum surface in less than 24 hours. The specimens of Test A1 failedafter 22 days exposure and all of the test specimens prepared byprocesses A3, A4, and A7 showed excellent stability with less thantwo-tenths inch of crack growth after 125 days of salt spray exposure.These tests indicate that the process of this invention is capable ofproducing a stable bond surface using wide ranges of acidconcentrations, potential and temperature. An upper and lowertemperature range is shown at which decreasing bond performance resultswhen temperatures in excess of about 95° F. are used and whentemperatures below about 60° F. are used. The optimum parameters appearto be the following:

    ______________________________________                                        Orthophosphoric acid                                                                             10% by weight                                              Potential          10 volts                                                   Time               20 minutes                                                 Temperature        75° F.                                              ______________________________________                                    

EXAMPLE VI

Samples of 2024-T3 bare aluminum alloy plate (an alloy containing about4.5% copper, about 0.6% manganese and about 1.5% magnesium) wereprepared for bonding in a 10% phosphoric acid anodization, using 10volts potential for 20 minutes at 70° F. The surfaces were primed withBR127 and samples bonded together with AF126 epoxy resin. Identicalspecimens were prepared using the H₂ SO₄ -Na₂ Cr₂ O₇ - 2H₂ O etchdiscussed above. Both sets of samples were exposed to 5% salt spray at95° F while the bond was placed under an initially high stress andmaintained under stressed conditions for an extended period of time. Atthe end of 70 days the samples prepared with H₂ SO₄ -Na₂ O₇ -2H₂ Ofailed adhesively over the entire length of the stressed bond. Thesamples prepared using H₃ PO₄ anodization exhibited no adhesive failureat the end of 18 months exposure. A cohesive failure crack extendedabout one half inch along the bond, exclusively with the adhesivematerial.

Various modifications and improvements can be made to the presentinvention without departing from the spirit thereof and from the scopeof the claims set forth below.

We claim:
 1. A method of preparing an adhesively bonded aluminumstructure comprisingforming a porous columnar aluminum oxide coating onsurfaces of aluminum alloy articles, said aluminum alloy articlescontaining about 1.6 to about 4.5% by weight copper by anodizing saidarticles in an aqueous solution comprising phosphoric acid, theanodizing potential being from about 3 to about 25 volts, the phosphoricacid concentration being about 3 to about 20% by weight and thetemperature of said solution being from about 50° F. to about 85° F.;rinsing said articles to remove said solution; applying an adhesive tosaid surfaces; and bonding said aluminum articles together to form saidadhesively bonded structure.
 2. The method of claim 1 wherein saidalumimum alloy article consists of aluminum alloy
 7075. --
 3. The methodof claim 1 wherein said aluminum alloy article consists of aluminumalloy 2024.--
 4. The method of claim 1 wherein said temperature range is75°±10° F.--
 5. The process of claim 1 wherein said porous oxide coatingis primed with an epoxy resin primer prior to application of saidadhesive.
 6. A process for preparing an adhesively bonded structure ofcopper-containing aluminum alloy adherends wherein said adherends have apolymer receptive aluminum oxide surface thereon having a porouscolumnar structure with a thickness of from about 500 to about 600Angstroms, having pores from about 300 to about 600 Angstroms indiameter and from about 400 to about 5000 Angstroms in depth extendinginto said oxide surface, said structure prepared by the stepsof:cleaning and deoxidizing said adherends; anodizing said adherends inan aqueous phosphoric acid solution containing from about 3 to about 20%by weight H₃ PO₄ at a temperature of from about 50 to about 85° F. forabout 10 to about 30 minutes at a potential of about 3 to about 25volts; rinsing said adherends to remove said aqueous phosphoric acidsolution; applying an adhesive to the surface of at least one of saidadherends; and bonding said adherends together to form said structure.7. The process of claim 6 wherein said adherends contain from about 1.6to about 4.5% by weight copper.
 8. The process of claim 6 wherein saidadherends consist of aluminum alloy
 2024. 9. The process of claim 6wherein said adherends consist of aluminum alloy
 7075. 10. The processof claim 6 wherein said temperature range is 75°±10° F.
 11. The processof claim 6 wherein said temperature is about 70° F, said solutioncontains about 10% phosphoric acid, said potential is about 10 volts andsaid anodization time is about 20 minutes.
 12. The process of claim 6wherein said oxide surface is coated with an epoxy primer and adhered toanother surface with an epoxy resin adhesive.
 13. The process of claim 6wherein said porous oxide coating is primed with an epoxy resin primerprior to application of said adhesive.
 14. An adhesively bonded alumimumalloy structure formed by adhesively bonding together aluminum adherendscontaining from about 1.6 to about 4.5% by weight copper, said adherendseach having a polymer receptive aluminum oxide surface thereon having aporous columnar structure with a thickness of from about 500 to about6000 Angstroms, having pores from about 300 to about 600 Angstroms indiameter and from about 400 to about 5000 Angstroms in depth extendinginto said oxide surface, said structure prepared by anodizing thesurfaces of said adherends in an aqueous phosphoric acid solutioncontaining from about 3 to about 20% by weight H₃ PO₄ at a temperatureof from about 50° F. to about 85° F. for about 10 to about 30 minutes ata potential of from about 3 to about 25 volts, rinsing said adherends toremove said solution, applying a polymeric adhesive to said surfaces andbonding said adherends together.
 15. The structure of claim 14 whereinsaid temperature range is 75±10° F.
 16. The structure of claim 14wherein said temperature is about 70° F., said solution contains about10% by weight phosporic acid, said potential is about 10 volts and saidanodization time is about 20 minutes.
 17. The structure of claim 14wherein said temperature is about 70° F., said solution contains about10% by weight phosphoric acid, said potential is about 10 volts and saidanodization time is about 20 minutes.
 18. The process of claim 14wherein said porous oxide coating is primed with an epoxy resin primerprior to application of said adhesive.
 19. A method of preparing anadhesively bonded aluminum structure comprising:forming a porous oxidecoating on surfaces of aluminum articles, said oxide coating being apolymer receptive, substantially unhydrated aluminum oxide having aporous columnar structure with a thickness of from about 500 to about6000 Angstroms, having pores from about 300 to about 600 Angstroms indiameter and from about 400 to about 5,000 Angstroms in depth extendinginto said oxide surface, by anodizing said articles in an aqueous acidicsolution, the acid component thereof consisting essentially ofphosphoric acid, the anodizing potential being from about 3 to about 25volts, the phosphoric acid concentration being about 3 to about 20% byweight and the temperature of said solution being from about 50° F. toabout 85° F.; rinsing said articles to remove said solution; applying anadhesive to said surfaces; and bonding said aluminum articles togetherto form said adhesively bonded structure.
 20. The method of claim 19wherein said temperature range is 75°±10° F.-
 21. The method of claim 19wherein said potential is 10±1 volts.
 22. The method of claim 19 whereinsaid phosphoric acid concentration is 10±2% by weight.
 23. The method ofclaim 19 wherein said anodization is conducted for from 10 to 30minutes.
 24. The process of claim 19 wherein said porous oxide coatingis primed with an epoxy resin primer prior to application of saidadhesive.
 25. An adhesively bonded structure formed by bonding togetheraluminum adherends, each adhered having a polymer receptivehydration-resistant, aluminum oxide surface thereon, said oxide surfacehaving a porous columnar structure with a thickness of from about 500 toabout 6,000 Angstroms, having pores from about 300 to about 600Angstroms in diameter and from about 400 to about 5,000 Angstroms indepth extending into said oxide surface, said surface prepared byanodizing the surface of said adherend in an aqueous acidic electrolyte,the acidic component of said electrolyte consisting essentially of H₃PO₄ present in amounts from about 3 to about 20% by weight H₃ PO₄ at atemperature of from about 60° F. to about 90° F. for about 10 to about30 minutes at a potential of from about 3 to about 25 volts, rinsingsaid adherend to remove said solution, applying an adhesive to saidsurfaces and bonding said adherends together to form said adhesivelybonded structure.
 26. The structure of claim 25 wherein said temperaturerange is 75°±10° F
 27. The process of claim 25 wherein said porous oxidecoating is primed with an epoxy resin primer prior to application ofsaid adhesive.
 28. A process for preparing an adhesively bonded aluminumstructure from aluminum adherends comprising the steps of:cleaning thesurface of said adherends; deoxidizing the surface of said adherends;forming a porous aluminum oxide surface on said adherends by anodizationof said adherends in an aqueous acidic solution, the acidic componentthereof consisting essentially of phosphoric acid, the anodizingpotential being from about 3 to about 25 volts, the phosphoric acidconcentration being about 3 to about 20% by weight, the temperature ofsaid solution being in the range of 75°±10° F.; rinsing said adherendsto remove said solution; applying an epoxy resin primer to said porousaluminum oxide surface; curing said primer; applying an adhesive to atleast one of said adherends; and bonding said adherends together to formsaid adhesively bonded aluminum structure.
 29. The product produced bythe process of claim 28.